(a) Field of the Invention
The present invention relates to a clock generator, and more particularly, to a clock generator that is substantially immune from the influence of manufacturing process, supply voltage, and temperature.
(b) Description of the Related Art
The compensation correction circuits of manufacturing process, voltage, and temperature are used to stabilize variables and control oscillator. The oscillator in a different environment can output accurate frequency, immune from the influence of environment and manufacturing process.
The general of compensation practice is to use another oscillator to detect changes in the environment, then the detection of the result is transmitted to compensate the circuit. This method requires a total of two oscillators which increase consumption of area and power. If these two oscillators mismatch, the detected result could be inaccurate, making the output frequency different from the design. In addition, the compensation circuit controls the current source to adjust the output frequency of the oscillator, but the current leakage of the transistor in the advanced manufacturing process would be a problem in this practice. It is more difficult to perform the matching between the two oscillators if the current of the current source controlled by the compensation circuit is extremely small.
Another compensation practice is to use differences of the temperature coefficient to compensate. In order to generate appropriate temperature coefficient, the hybrid concentration of electronic and electric hole should be adjusted during the production process. In the actual production of on the need more than one procedure will increase production costs. During the production process, if the process has drift, it cannot produce the correct temperature coefficient, it will lack repression to process variations. Therefore, the oscillator is made from the charge and discharge of resistors and capacitors, each resistor will need a positive temperature coefficient of the resistor and a negative temperature coefficient of the resistor, to connect in series to couple to offset the impact of temperature on the oscillation frequency. However, if the production in the process drifts, then the resistor will still be affected by temperature, so the precision of output frequency will be reduced. If the oscillator couples more groups such resistors, the repression to process variations will be worse. At low frequency, the output frequency precision is more easily influenced by temperature because of the requirement of the larger resistor.
However, there uses the bandgap reference voltage regulator to have stable voltage to suppress the impact of voltage variability, and reduces effects from temperature and process variation with internal use of the temperature/process circuit. For high precision of the output frequency, the two circuits need more power consumption of several mA. If it is used for low power applications, it can not achieve high precision output.